"Think about a block of clay that's been sitting there," says Polly de Mille, R.N., exercise physiologist and coordinator of performance services at the Tisch Sports Performance Center at Hospital for Special Surgery in New York. That cold block of clay would tear if you stretched it, compared to how pliable it would be if you spent some time warming it up in your hands first, she says. Our muscles and connective tissue also have less elasticity when the temperature gets lower, she says.

That's why warming up is more important than at any other time of year, she says. In average temps, when you're not using your muscles, most of your blood flows to your internal organs. When you start to call on your legs and arms to get moving, blood vessels open up to fuel those working muscles, she says. But when the mercury drops, "you're amplifying that effect," she says. If you jump right into a sudden, powerful movement, like sprinting, on a stiffer-than-normal muscle, that force could lead to injury.

The cold may also slow down some of our "sensory mechanisms", she says. "Your nerves are colder, so there's slower transmission rate," making, say, your feet a little numb, which could throw off your balance, she says. It's possible, then, to be doing damage without being totally aware of it. In warmer weather, you might read a twinge of pain as a signal to ease up; in cold weather, you might push yourself through the twinge toward injury.

The good news is cold-weather exercise injuries are preventable. "If you're dressed appropriately for the weather and you do a gradual, proper warmup, you can avoid a lot of that," she says. "Look at the warmup as literally warming up the muscle, tendon and other parts of your body to get ready for the greater forces that you'll be applying to them in sprinting or jumping or landing."